The prior art is particularly illustrated in European patents EP-A-0 688 590, EP-A-0 415 822, EP-A-0 075 611 and U.S. Pat. No. 5,156,736.
In simulated moving bed separation processes such as those carried out in the "Sorbex" series of processes, among them the PAREX.RTM., MOLEX.RTM., SAREX.RTM., OLEX.RTM., and EBEX.RTM. processes, a plurality of beds are used which are localised in one or two adsorption columns. Each distributor plate situated between two consecutive beds is connected to the exterior by means of a single line leading into a rotary valve which brings each of the beds in succession into communication with each of the streams entering or leaving the adsorption section in sequence. Such streams comprise:
1) the feed to be separated constituted by a mixture of at least two products: PA1 2) the solvent or desorbent which elutes or desorbs the constituents of the feed; PA1 3) the extract, constituted by a mixture of the most adsorbed product (A) and the desorbent; PA1 4) the raffinate, constituted by a mixture of the least adsorbed product (B) and desorbent; PA1 5) the in flush, or in rinse, constituted by a mixture of extract and desorbent, which can flush the slug of feed trapped in the common line after the feed has been introduced into the adsorber into the interior of the adsorber; PA1 6) the out flush, or out rinse, constituted by a mixture of extract and desorbent, which draws the slug of extract which is trapped in the common line after the extract has been extracted from the adsorber toward the exterior. The in flush and out flush flow rates are equal, and a pump places the out flush stream in communication with the in flush. The flush flow rate is calculated so that the volume of the longest line connecting the rotary valve to the furthest bed is flushed 2 to 3 times during a permutation period; PA1 7) the secondary flush can be constituted either by desorbent, or by extract which is depleted in desorbent. Its aim is to flush the extremity of the common line so as to flush any impurity which may have lodged there by diffusion or exchange into the interior of the adsorber just before extracting the extract.
A the most adsorbed in the beds and B the least adsorbed, or retarded, in the beds;
The disadvantage of this type of process is that each of the common lines must be flushed between introducing the feed and extracting the extract and between extracting the extract and introducing the desorbent, if a high purity of constituent A is desired in the extract. The flush flow rate linked to the highest volume of flushed line is far from negligible in the light of the feed flow rate, and it has the effect of causing the system to operate slightly off the optimum flow rate in the different zones.
A further disadvantage of coupling by means of the rotary valve of the in flush and out flush is that this requires a pump, a flow meter and a flow rate regulating valve since during a cycle, the pressure of the out flush can easily be lower than the pressure of the in flush.
Further, the flow rate regulating system in the in flush, out flush loop is not particularly suitable for a programmable flow rate which varies, for example, from zero over a certain portion of the period to a certain reference value during another portion of the period, thus allowing effective flushing with a minimum displaced volume.
An alternative technique which is used in the Eluxyl process, for example, consists of connecting each distributor plate located between two consecutive beds to the exterior by at least two distinct circulation or distribution lines.
It also contains a distinct on-off valve per distributor plate and per principal entering or leaving stream (desorbent, extract, feed, raffinate).
In principle, if one of the two lines is used for "clean" fluids (desorbent or extract), and the other is used for "dirty" fluids (feed or raffinate), flushing each of the two common lines becomes superfluous. If not just two lines dedicated to "clean" and "dirty" fluids are used, but four distinct lines are used each connecting each of the principal streams to the distributor plate, flushing such lines is in principle of no use.
However, each of such lines leads into the principal stream circulating from one bed to the next and the extremity of the two lines (dedicated to clean and to dirty fluids) or the extremity of the four lines dedicated to extract, raffinate, feed or desorbent may be contaminated by exchange or diffusion with the principal fluid. When the purity and yield are to be maximised, such contamination becomes deleterious.